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Extension > Agriculture > Manure Management and Air Quality > Manure Application > Steps for calculating rates

Steps for calculating rates

(Original material from Custom Manure Applicator Certification Manual 2002 adapted for web page)

Introduction

Each time manure is spread on a field an application rate must be determined (in tons or gallons of manure per acre). This application rate is calculated using three basic steps, which will be developed in more detail later.

  1. Determine the nitrogen (N) and phosphorus (P) recommendations or removal rates for the crop to be grown;
  2. Determine the nitrogen credits from previous legume crops and manure applications; and
  3. Determine the plant-available nitrogen and phosphorus in the manure to be applied.

Proper manure application means that the nutrients supplied by the manure meet, but do not exceed, the nutrient demands from the field. Under current regulations the maximum allowable rate for manure application is usually based on crop nitrogen needs for non-legumes and on crop nitrogen removal for legumes. However, it is advisable to use P-based rates near surface waters and conduits to surface waters with potential for runoff and erosion and to consider P-based rates in all fields.

If a manure management plan has been completed, the following information should be noted in the plan:

If no manure management plan has been developed, this information must be determined before applying the manure.

Application Rate Worksheet

Click the graphic on the right to view and print the rate calculation worksheet.

Use the empty boxes in the right column of the worksheet to enter the information you need to calculate an application rate. The calculations in the "Example" column are based on the following farm profile:

STEP 1

Determine the N and P recommendations for the crop to be grown.

If N recommendations have already been determined (and include N credits from manure and legumes) enter those values in boxes 2h and proceed with Step 3. To determine the nitrogen recommendations and phosphorus removal rates for non-legume crops, use Table 1. To use Table 1, you need to know the yield goal, previous crop, and soil organic matter level. The farmer should be able to supply this information. Use Table 2 to determine nitrogen and phosphorus removal rates for legumes.

STEP 2

Determine the amount of nitrogen still needed to meet the crop needs by subtracting the nitrogen remaining from last year’s manure application or the previous year’s alfalfa (second-year manure and alfalfa credits).

To determine the amount of nitrogen available from the previous year’s manure application (second year credit), you must know the previous year’s manure application rate and nutrient content along with the second-year nitrogen availability (in percent). Farm records should provide the application rate (in gallons or tons per acre) and nitrogen content. If the nitrogen content of the applied manure is unknown, use values listed in Table 3. The following percentages are for second-year nitrogen availability for the various types of manure:

For this example, alfalfa was plowed down prior to growing the previous crop, therefore, a significant amount of nitrogen may remain in the soil. Table 4 lists the pounds of nitrogen available from this second-year alfalfa credit.

The final calculation in this step subtracts the manure credits and second-year alfalfa credits from the original nitrogen recommendation. This step gives the amount of additional nitrogen that is needed to meet the needs of the crop.

STEP 3

Determine the available N and P of the manure.

Manure nutrient content depends on the species and size of animal and the manure handling system and are highly variable from year to year. Therefore, it is best to estimate the N and P content from several years’ manure test results (average content over several years of sampling). If such historical data is not available, refer to Table 3.

The availability of N is based on the species of animal and the application method. From Table 5 select the percent of total nitrogen available to the crop during the growing season. Nitrogen availability is calculated by multiplying the nitrogen content of the manure by the percent availability.

The availability of P2O5, is estimated at 80% of total phosphorus in the manure.

Note: A “quick” manure test gives the inorganic nitrogen content and cannot be used with any of the calculations in this worksheet.

STEP 4

Calculate the manure application rate based on nitrogen recommendations.

The manure application rate is calculated by dividing the additional amount of nitrogen needed by the crop by the amount of nitrogen available from the applied manure.

STEP 5

For areas where phosphorus runoff or soil erosion is of concern, calculate the amount of phosphorus applied that will be used by subsequent crops.

When calculating application rates based on nitrogen, as we have done thus far, phosphorus will usually be applied in excess of the crop recommendation. This phosphorus buildup near surface waters and pathways to surface waters is an environmental concern. Follow MPCA guidance for applying manure in these areas. (MPCA Publication “Applying Manure in Sensitive Areas”)

Table 1. Nitrogen fertilizer recommendations as a function of previous crop, anticipated yield goals, and organic matter
Table 2. Nutrient removal from legumes and corn silage
Table 3. Nutrient content of stored manure
Table 4. Second-year alfalfa credits
Table 5. Manure nitrogen first-year availability as affected by manure application method and manure type

 

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